The present invention relates an automatic control method and apparatus. More particularly, the present invention relates to a method and apparatus for controlling the speed of an internal combustion engine by using a pulse width modulator (xe2x80x9cPWMxe2x80x9d) to drive a magnetic linear actuator.
Small internal combustion engines (xe2x80x9cIC enginesxe2x80x9d) are lightweight and inexpensive power sources. These features make small IC engines an attractive choice for portable electric generators. These generators are commonly used to provide electric power in places without access to the national electric grid, and are particularly popular for use on construction sites, in recreational vehicles in remote areas, and during power outages.
One problem with the use of IC engines in portable generators, however, is that many electrical appliances require alternating current at almost exactly 60 hertz. Specifically, current specifications require a frequency variance of about xc2x13 to 5 hertz without load and while loading, and a steady state frequency variance of about xc2x10.6 to 0.8 hertz under load. Meeting these specifications requires that the speed of the IC engine be very accurately controlled.
A conventional solution to this speed control issue is to use a mechanical governor. One such governor slidably attaches a fan blade to the engine""s output shaft. As the motor accelerates, the fan begins to generate an axial force. This axial force biases the fan blade against a spring. The resulting relative motion is related to the fan""s angular velocity and can be used to actuate the engine""s throttle position. Another type of governor pivotally attaches weights to a rotating shaft. The resulting centripetal force pivots the weights radially outward against gravity or against a spring. The angle between the weights and the shaft is related to the shaft""s angular velocity and is used to actuate the engine""s throttle position.
Although mechanical governors are relatively inexpensive, they generally respond slowly to changes in the engine""s load. This problem is particularly burdensome in portable generator applications because many common electrical loads (e.g., heaters, hair dryers, and incandescent lamps) are applied and removed instantaneously. This instantaneous change in load, combined with the mechanical governor""s slow response time, can result in unacceptable deviation from the desired frequency.
One partial solution to this response time problem is to reduce damping within the governor. This solution, however, can lead to overshoot and undershoot problems, and other unacceptable variations. Another partial solution to this response time problem uses a small electric motor to control a throttle valve. This system, however, is complex and expensive, which makes it uneconomical for use in the small portable generators.
Clearly, there is a need for a cost-effective control method and apparatus that can maintain a constant engine speed and that can rapidly respond to load changes with minimal overshoot or undershoot. There is also a need for a speed control device that is capable of proportional, integral, or differential control of a single or a multi-cylinder IC engine.
The present invention provides a cost-effective controller that can maintain a constant engine speed and can rapidly respond to load changes with minimal overshoot or undershoot. One embodiment generally comprises a controller and a linear actuator. The controller generates a plurality of voltage pulses having a duration and a frequency related to the difference between a desired engine speed and an actual engine speed. The linear actuator in some embodiments comprises of a magnet associated with an actuator rod and a solenoid coil. The plurality of voltage pulses generate a current in the solenoid coil, which creates a magnetic field. The magnet interacts with magnetic field interacts to generate an actuating force. This actuating force biases the actuator rod in a first direction.
Some embodiments of this invention enclose the linear actuator in a ferrous metal housing. Hysteresis effects in this housing generate a return force that biases the actuator rod in a second direction, opposite of the first direction. This return force will cause the throttle to automatically close in the event of a power failure, thereby creating an automatic fail safe feature. Still other embodiments of this invention replace the ferrous metal housing with a housing made from an appropriate nonferrous material, such as a plastic, and use a return spring to close the throttle.
Another embodiment of the present invention comprises a controller operatively connected to an engine speed sensor and adapted to produce a signal related to the difference between an actual engine speed and a desired engine speed; a pulse width modulator that generates a plurality of voltage pulses having a duration and frequency related to the signal from the controller; and a linear actuator assembly that converts the plurality of voltage pulses into a throttle position. The linear actuator assembly, in turn, comprises a solenoid coil, electrically coupled to the pulse width modulator, that generates a linear actuation force during the plurality of voltage pulses, wherein the linear actuation force translates an actuator rod in a first direction; a linkage that couples the actuator rod to a throttle valve; and a biasing element adapted to generate a return force between the plurality of voltage pulses, wherein the return force translates the actuator rod in a second direction.
Another aspect of the present invention is a method of controlling engine speed. One embodiment of this method comprises generating a plurality of voltage pulses having a duration and a frequency related to a difference between a desired engine speed and an actual engine speed, wherein the plurality of voltage pulses drive a linear actuator; and actuating a throttle valve with the linear actuator. The method may further comprise generating a pulse width counter value and a terminal value; establishing a counter for storing values used in performing iteration; setting the counter to the pulse width counter value; iteratively decrementing the counter while the counter is greater than the terminal value; and changing an output state of a pulse width modulator.
One feature and advantage of the present invention is its low cost. This feature allows it to be economically used to control small portable generators. Another feature and advantage is that a fail safe feature automatically shuts the IC engine off in the event of a power failure, or other malfunction, in the control circuitry. Still another feature of the present invention is that it minimizes the amount of hardware necessary for implementation, which reduces both board real estate and component costs. These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings.